Recovery of furfural by preferential absorption of the vapor on solids



RECOVERY OF FURFURAL BY PREFERENTIAL ABSORPTIUN OF THE VAPOR N SOLIDS Oliver W. Cass, Lewiston, N. Y., assignor to E. I. du Pont de Nemonrs and Company, Wilmington, Deh, a corporation of Delaware No Drawing. Application February 23, 1954, Seriai No. 412,115

10 Claims. (Cl. 266-3473) This invention relates to furfural and more particularly to the recovery of furfural from admixture with steam.

Furfural is commercially manufactured by the acid hydrolysis of pentosans. In typical procedures oat hulls, corncobs, bagasse, hard wood and similar vegetable prodnets of a high pentosan content are heated under acidic conditions to yield a mixture containing the desired furfural. Steam at atmospheric or elevated pressure is ordinarily used for heating the raw material directly. The furfural is, therefore, most conveniently steam distilled from the non-volatile cmponents of the mixture as it is formed.

Since the reactions converting the pentosans to furfural are rather slow, relatively large amounts of steam are needed particularly to maintain the proper temperature during the reaction cycle. The quantity of steam re quired is increased further by the necessity for rapidly removing furfural from the reaction mixture producing it to avoid side reactions. The concentration of furfural in the product vapors is accordingly quite low. Recovery of the low concentration furfural by multiple dist.llation procedures thus becomes in turn another operation consuming excessive amounts of steam. U. S. Patents 1,735,084 and 2,140,572 illustrate processes of the type under consideration.

Many proposals have been advanced for reducing the quantity of steam used in the production of furfural and for increasing the concentration of this compound in the product vapors. Despite these proposals, total steam consumption in the processes generally used for manufacturing furfural remains very high, actually of the order of fifteen to thirty pounds of steam for each pound of refined furfural, The cost of this steam is thus a major factor in the price of furfural as it is now commercially produced.

A general object of my invention is, therefore, provision of an improvement in the process for making furfural. Another object of the invention is to reduce the quantity of steam presently needed in the manufacture of a given quantity of furfural. An additional object is development of a method for separating furfural vapor from steam without condensation of the steam. A further object of the invention is development of a method for increasing the concentration of furfural in the vapors obtained from the steam distillation of an acidified pentosan-containing reaction mixture.

The above-mentioned and still further objects of the invention may be accomplished by a procedure in which pentosans are hydrolyzed and then steam distilled with steam at a relatively lowpressure in the usual fashion. The steam from the distillation is, however, not condensed but is passed through activated charcoal or other furfural absorbent. Care is taken that the temperature of the absorbent is suiliciently high to avoid condensation of the steam. Passage of the steam through such an absorbent will remove the furfural it contains even at temperatures of 100130 C. Steam stripped of its fur- 2,779,770 Patented Jan. 29, 1957 ice at fural content by passage through an appropriate absorbent may be heated slightly, if necessary, and recycled through the hydrolysis zone. When the absorbent employed has taken up a maximum or at least an appreciable percentage of furfural, the latter may be desorbed in any convenient manner. In the preferred mode of accomplishing this invention, however, the furfural is removed by passing steam at a relatively high pressure through the absorbent. This passage of high pressure steam through the absorbent produces a vapor mixture containing much more furfural than the low pressure mixture produced in the steam distillation. The high pressure mixture actually contains so much furfural that when it is condensed it readily divides into furfural and water layers separable by decantation.

Details of the invention will be evident from the following example:

A conventional insulated absorption column was packed with 35 parts by weight of an active carbon absorbent and, together with its packing, heated with steam to approximately 115 C. Low pressure steam at 115 C. containing 2 parts by weight of furfural in each parts of steam was admitted at one end of the column and passed therethrough. This low pressure steam was the equivalent of that obtained by steam distillation of an acidified pentosan mixture in the normal production of furfural. The flow of the low pressure steam containing furfural was maintained for several minutes. Substantially no furfural was detectable in the steam issuing from the column until about 66 700 parts had passed through. When furfural became detectable in the off-gas,

the flow of low pressure steam was diverted from the column and relatively high pressure steam, free from furfural, was rapidly admitted to the column. The major portion of the absorbed furfural was removed from the absorbent in such concentration that, upon condensation, a separate decantable layer of furfural was obtained. Furfural was found to comprise more than 10% by weight of the total condensate.

It will readily be apparent that the low pressure steam shown above may be recycled to the hydrolysis Zone as desired by use of an appropriate combination of heat exchangers and blowers. Recirculation of this steam will clearly reduce the amount needed to distill the furfural to that required to fill the reaction system under the conditions of temperature and pressure prevailing within that system. At no time is a substantial quantity of the steam used in the hydrolysis itself condensed. Heat requirements for the reaction are thus held to a minimum. Since very little high pressure steam is needed to flush out the absorbent when it becomes saturated with furfural, heat requirements for the furfural recovery are also minimized.

The foregoing example and discussion clearly show that the objects of the invention set forth have been achieved. It may be particularly pointed out, however, that the concentration of furfural in the final product steam has been increased to over five times that in the feed steam by use of the preferred mode of separating furfural from the absorbent by use of high temperature steam. In fact, a decantable furfural phase is obtained upon condensation of the high pressure steam.

In continuous operation of this invention, I prefer to employ two absorbers in parallel. Thus, While one absorber is removing furfural from steam, the other can be stripped. The invention may also be applied to a multiple as well as to a single reactor system. In the latter case, stripped steam is, of course, recycled repeatedly through the same reactor. In the former case, however, stripped steam may be passed through asecond reactor and even a second absorber if it is so desired. Other combinations of reactors and absorbers utilizing the principles of this invention will be evident to those skilled in the art.

The temperatures required for the absorption and desorption steps are not sharply critical. it is essential, however, that there be an appreciable temperature differential between the two steps. In practice, it is found that a minimum differential of about 30 C. is necessary for good results. The minimum temperature for the absorption step must lie above the condensation point of steam at the pressure involved. Generally, an absorption temperature in the range 100-130 C. will be found satisfactory, with 115-120 C. being about optimum. The upper limit for the desorption step is set chemically by the temperature at which side reactions involving furfural begin to appear and physically by the economic considerations involved in the construction of pressure equipment. A satisfactory upper temperature limit for the desorption step is found at about 180 C. Thus, according to the principles of this invention, the desorption step will generally be carried out at between about 130 and 180 C. with at least about 150 C. being preferred. Pressures need be no higher than are required to maintain the absorption and desorption temperatures chosen. The autogenous pressures developed by the steam at the predetermined temperatures are preferred.

The carbon utilized in the example given was the wellknown activated charcoal Nuchar. Other absorptive carbons or charcoals may also be used. in addition, other porous absorptions agents such as alumina, silica gel and the like may be substituted for varieties of carbon.

The concentration of furfural in the feed steam from the acid hydrolysis zone will vary to some extent but will never be very high. The maximum furfuraf content of this steam will generally not exceed 5-6%. Since the distillation must be carried out rapidly, as low as 1-2% furfural may be carried over. It is an advantage of this invention, in fact, that it can handle steam of a low furfural content.

The steamzabsorbent volume ratio will vary with the absorbent, the concentration of furfural in the steam and with the temperature of absorption. For Nuchar and steam containing about 2% furfural at 11S-120 C., the ratio for complete absorption cannot be greater than about :1 as shown. It is obviously unnecessary that the absorbent be completely saturated with furfural in the absorption step or entirely stripped in the desorption step.

. The enrichment of steam with furfural vapors according to this invention is not restricted to processes involving the manufacture of the compound. Thus dilute aqueous solutions of furfural may be vaporized and passed through an absorbent. Subsequent desorption of the selectively absorbed furfural with steam yields steam enriched as in the example above. Selective absorption of the furfural may also be accomplished from vapors containing organic compounds in addition to fuifural. It is merely essential that the additional vapors do not interfere with the furfural absorption.

If the objects of the invention be restricted primarily to the recovery of furfural from dilute admixture with steam, the absorbent need not be steannstrlpped in the desorption step. Furfur'al may, for example, be removed by extraction with low boiling solvents such as tetrahydrofuran. Atlernatively, the furfural can be distilled directly from the absorbent with application of heat alone. The use of an inert gas other than steam in the stripping operation is also feasible. The procedure which accomplishes all the objects of the invention is, however, that given in detail above, a procedure in which low and high temperature steam are alternately passed through the absorbent.

. The procedure exemplified is consequently preferred.

steam distillation, the steps comprising passing the resultant vapor mixture of steam and furfural through a suitable solid absorbent for furfural at a temperature of about -130 C. and thereby absorbing the furfural from the steam and subsequently recovering the furfural from the solid absorbent.

2. In the manufacture of furfural by the hydrolysis of a reaction mixture of acidified vegetable matter containing pentosans and recovery of the furfural therefrom by steam distillation, the steps comprising passing the re sultant vapor mixture of steam and furfural through an absorbent of the group consisting of active carbon, alumina and silica gel at a temperature of about 100-130 C. and thereby absorbing the furfural from the steam and subsequently recovering the furfural from the solid absorbent.

3. in the manufacture of furfural by the acid hydrolysis of a pentosan mixture and recovery of the furfural therefrom by steam distillation, a first step of continuously distilling furfural from said' mixture by the use of steam at a temperature of about 100-130 C., a second step of continuously passing said steam through an absorbent of the group consisting of active carbon, alumina and silica gel at 100-130 C. and a third step of interrupting the second step and passing through the absorbent steam initially containing no furfural at a temperature of between 130 and 180 C. but at least 30 C. above the temperature of the steam used in the second step and thereby removing the furfural from said absorbent at a concentration greater than its concentration in the steam oi the second step.

4. The invention of claim 3 in which the steam stripped of furfural in the second step is recycled to the pentosan mixture and the furfural is subsequently separated from the steam resulting from the third step.

5. The invention of claim 3 in which the steam from the third step is condensed, the condensate is allowed to divide into a furfural and an aqueous layer and the two layers are separated by decantation.

6. In the manufacture of furfural by the decomposition of pentosans with acid, the improvement which comprises removing product furfural from said acid by steam-distillation with low-pressure steam, passing the vapors from the steam-distillation through activated carban to absorb the furfural thererrom, recycling the steam thus freed from furfural to the steam distillation, interrupting the passage of the vapors through the carbon, recovering the furfural from the carbon during the interruption by passing through the carbon high-pressure steam at a temperature greater than previously employed, condensing said high-pressure steam after its passage through the carbon and separating furfural from the condensate.

7. The process of claim 6 in which the temperature of the low-pressure steam is about -120 C. and the temperature of the high-pressure steam is about 150-180 C.

8. The process which comprises distilling t'urfural from a dilute'solution thereof with low-pressure steam at a temperature of about 100-130 C., passing the resultant vapors through activated carbon to absorb the furfural therefrom, subsequently passing high-pressure steam at a temperature of about -180 C. but at least 30 C. greater than that of the low-pressure steam through said carbon, condensing said high-pressure steam and separating furfural from the condensate.

9. The process of claim 8 in which the temperature of the low-pressure steam is about 115-120 C. and the temperature of the high-pressure steam is about -180 C 10. The method of recovering f urfural from vapor phase admixture with steam without condensing the steam which comprises contacting said admixture with activated carbon at autogenous pressure and a temperature above the vaporization temperature thereof, the admixturemarbon volume ratio being not greater than about 20: 1.

(References on following page) References Cited in the file of this patent 2,369,655 UNITED STATES PATENTS 1,322,054 Ripard Nov. 18, 1919 215591607 1,735,084 Mlner et a1 Nov. 12, 1929 5 1,889,617 Truog Nov. 29, 1932 2,140,572 Brownlee Dec. 30, 1938 6 Boehm Feb. 20, 1945 Claussen et a1 May 17, 1949 Hartig Aug. 8, 1950 Dunning et a1. July 10, 1951 OTHER REFERENCES Chem. Abst., vol. 42, col. 223g (11948). 

1. IN THE MANUFACTURE OF FURFURAL BY THE HYDROLYSIS OF A REACTION MIXTURE OF ACIDIFIED VEGETABLE MATTER CONTAINING PENTOSANS AND RECOVERY OF THE FURFURAL THEREFROM BY STEAM DISTILLATION, THE STEPS COMPRISING PASSING THE RESULTANT VAPOR MIXTURE OF STEAM AND FURFURAL THROUGH A SUITABLE SOLID ABSORBENT FOR FURFURAL AT A TEMPERATURE OF ABOUT 100*-130*C. AND THEREBY ABSORBING THE FURFURAL FROM THE STEAM AND SUBSEQUENTLY RECOVERING THE FURFURAL FROM THE SOLID ABSORBENT. 